a b s t r a c tThe effect of nickel on propane oxidation and sulfur resistance of Pt/Ce0.4Zr0.6O2 catalyst has been studied.Samples were characterized by X-ray diffraction, BET area, H2 temperature programmed reduction. It hasbeen found that the introduction of nickel not only enhances the surface area of the catalyst, but also decreases its reduction temperature. The nickel promoted catalyst is more active in complete oxidation ofC3H8. Furthermore, the addition of nickel to the catalyst is able to improve the desorption amount of sulfurspecies under reducing atmosphere, which could decrease the accumulation of sulfur species in the catalyst.Consequently, the sulfur resistance of Pt/Ce0.4Zr0.6O2 catalyst has been improved. 2013 The Authors. Published by Elsevier B.V. Open access under CC BY-NC-ND license.

1. IntroductionCeriazirconia (CZ) materials are widely used in the three-wayautomotive catalytic converter (TWC) as support material and as anoxygen promoter [1]. Its capacity of oxygen storage can adjust therange of air to fuel ratio approach to the narrow stoichiometricamount for simultaneously removing carbon monoxide, hydrocarbons and nitrogen oxides during rich/lean excursion of the exhaustgas [2]. Although current technologies of the TWC can meet the legislative emission requirements [3], history suggests that further developments will take place in the future as even tighter legislations(EURO6, LEVII, and J-SULEV) are introduced. These strict restrictionsnot only demand higher catalytic activity at lower temperatures, butalso require the mile durability mandated as 120,000 miles onwardsor more [4]. Thus, on the one hand, different fundamental studieshave been carried out on forming coating materials [58] withimproved high catalytic activity and thermal stability. On the otherhand, with the mile durability mandated increases, the sulfur, presentin all commercially available fuels, poisoning the catalysts is increasingly outstanding even with 30 ppm sulfur content. Therefore, theresearch on how to improve sulfur resistance of the TWC becomesan important signicance. For example, Machida et al. [9] have

reported that TiO2ZrO2 can be used as a NOx adsorbent having

improved tolerance to SO2. Corbos et al. [10] have investigatedthe inuence of both the support oxide and the barium loading ofPt/Ba/Support model catalysts on sulfur resistance. In the presentpaper, we have investigated the effect of introducing nickel into theCZ on propane oxidation and sulfur resistance of the obtained Ptcatalyst.2. Experimental2.1. SynthesisCZ with cerium:zirconium mol ratio of 4:6 was prepared bycoprecipitation. ZrOCl28H2O and (NH4)2Ce(NO3)6 were dissolved together in deionized water to form a 0.1 M solution salt. The solutionwas slowly added, with stirring, via a peristaltic pump to an excessof 0.2 M NaOH solution. After the addition was completed, the pHwas adjusted to 11 and the suspension was reuxed at 100 C for48 h in a Teon ask. The precipitate was centrifuged, washed,dried at 110 C and calcined at 600 C for 4 h. A heating ramp of5 C/min was used to reach the nal temperature. The synthesizedsample is labeled as CZ. The addition of nickel (3 wt.%) was done byincipient wetness impregnation using an aqueous nickel nitratesolution, and the resulting sample was dried at 110 C for 24 h andcalcined at 500 C in air for 3 h, labeled as Ni/CZ.Pt catalysts with Pt content of 0.5 wt.% were also prepared byincipient wetness impregnation method, in which H2PtCl6 solutionwas used as Pt precursor. Then, the catalysts were dried at 110 Cfor 24 h and calcined 500 C in air for 3 h. The resulting catalystswere signed as Pt/CZ and Pt/Ni/CZ.

1566-7367 2013 The Authors. Published by Elsevier B.V. Open access under CC BY-NC-ND license.http://dx.doi.org/10.1016/j.catcom.2013.04.022

Y. Zheng et al. / Catalysis Communications 39 (2013) 14

2.2. CharacterizationThe crystalline phases of the samples were determined by powderX-ray diffraction using a Phillips X' Pert diffractometer equipped witha copper anode. Nitrogen adsorption isotherms and textural propertiesof the samples were determined at 196 C using N2 nitrogen in aconventional volumetric technique by Micromeritics Tristar 3000. Hydrogen temperature programmed reduction (H2-TPR) measurementswere carried out to estimate the reducibility of the samples usingMicromeritics AutoChem II 2920 chemisorption instrument. Eachsample (50 mg) was pretreated at 500 C for 30 min in argon andcooled down to room temperature. The gas ow was then switchedto 10% H2/Ar, and the temperature was raised to 800 C at a rate of10 C/min. The consumption of H2 was monitored using a thermal conductivity detector (TCD). SO2 absorption and desorption were carriedout using Micromeritics AutoChem II 2920 chemisorption instrument.In a typical measurement, 50 mg of sample was heated to 300 C andhold for 60 min in N2 (30 mL/min), then 0.50 mL of SO2 (0.2% in N2)was pulsed every 5 min for 5 times. Conventional TPR/TPD in a streamof H2 (10% in Ar), and N2 was also carried out with a ow of50 mL/min. The reaction products, including H2S and SO2 releasedduring temperature programmed measurements, were determinedby means of mass spectrometry (Omnistar GDS-300 Balzers). Energydispersive spectroscopy (EDS) analysis was performed using EDAXFalcon energy dispersive spectrometer.

are highly homogeneously distributed in the as-prepared samples.

No NiO diffraction peak is observed in the XRD pattern of Ni/CZ.This is mainly owing to the high dispersion of NiO on the support orlow (3 wt.%) loading of Ni on well developed surface of CZ [6]. Fig. 2shows the N2 adsorption desorption isotherms of CZ and Ni/CZ.Both of the adsorption and desorption isotherms abruptly changewhen the relative pressure is in the range of 0.60.9, which is acharacteristic feature of mesoporous materials. And the hysteresisloop of H3 suggests that the obtained samples possess an irregularmesoporous structure [12]. Additionally, above the relative pressureof 0.9, the isotherms still rise, which indicates that the samples alsohave some macroporous structure [13]. The BET measurementsshow that the surface areas of CZ and Ni/CZ are 91.1 and108.6 m 2/g, respectively. Obviously, the introduction of nickelenhances the surface area of the CZ. It is probably more related tothe smaller NiO nanoparticle which is highly dispersed in the sample.The BJH method was employed to calculate the pore size distributionsof the samples (inset Fig. 2). CZ and Ni/CZ samples have almost thesame narrow pore radius size distribution centered on around3.5 nm, and pore radius of the latter slightly shifts to a small one.This may be due to the added Ni should be evenly distributed onthe CZ, which indicates that the adding 3 wt.% of Ni would not affectthe pore radius size distribution of the CZ, either. Obviously, theresults of XRD and BJH agree with each other.3.2. The reducibility of the samples

2.3. Catalytic activity measurement

The evaluation of catalyst activity was carried out using a xedbed continuous-ow reactor packed with 0.1 g of catalyst. Beforethe activity test, the catalysts were reduced at 500 C for 2 h inowing 10%H2/Ar (30 mL/min). The complete oxidation of C3H8over the catalysts was conducted in the stream of the feedgas mixture with a composition of C3H8/O2/N2 = 0.2/1.0/98.8 (molar ratio)at GHSV = 40,000 h 1. The reactants and products were analyzedby an on-line GC equipped with a FID. SO2 poisoning of the catalysts:the catalysts were exposed to feeding with 100 mL/min ow SO2(0.2% in N2) at 300 C for 80 h, the sulfur poisoned catalysts werelabeled as SPt/CZ and SPt/Ni/CZ.3. Results and discussion3.1. Structural and textural characterizationFig. 1 shows the XRD diffraction patterns of CZ and Ni/CZ, both ofthe patterns include the typical reections of the uorite structure [11]. In any cases, no lines corresponding to either ZrO2 orCeO2 can be observed, which clearly indicates that Ce and Zr ions

Fig. 1. The X-ray diffraction patterns of the CZ and Ni/CZ samples.

Fig. 3 shows H2-TPR proles of CZ, Ni/CZ, Pt/CZ and Pt/Ni/CZ. ForCZ, the reduction peak (500600 C) appears in its TPR prole canbe ascribed to the reduction of subsurface and surface oxygen of theCZ [14]. For Ni/CZ, this peak is shifted to lower temperature by thepresence of Ni, suggesting that the introduction of Ni promotes thereduction of CZ. Moreover, the low temperature peak (250400 C)attributed to the reduction of NiO [15]. Pt/CZ catalyst shows twopeak maxima at ca. 210 and 358 C, which can be associated withthe reduction of PtO species and formed on the interaction betweenPtO and the support [6], respectively. For Pt/Ni/CZ catalyst, the reduction peak with high intensity appears at 173 C and a shoulder at205 C, which can be associated to the reduction of PtO and NiO species, respectively. Additionally, the reduction peak maxima at ca.330 C can also be ascribed to the formed on the interaction betweenPtO and the support. We have known that the introduction of Nienhances the surface area of CZ. It suggests that the presence of Nipromotes the reduction of the PtO species highly dispersed on thesurface of the support. The reduction peak of NiO also shifted tolower temperature, which may be attributing to the strong interaction between Ni and Pt noble metals [16].

Y. Zheng et al. / Catalysis Communications 39 (2013) 14

3.3. SO2 absorption and desorption performance of the samples

The SO2 absorption and desorption treatments of samples werecarried out according to the previous work [17]. The sulfur poisonfeeding was absorbed completely for all samples. Meanwhile, theabsorbed SO2 would not desorb below 900 C under pure N2atmosphere, suggesting that all samples have strong adsorptioncapacity for the SO2. The species formed by exposure of ceria to SO2have been characterized by X-ray absorption spectroscopy (XANES),temperature programmed desorption (TPD), and high resolutionX-ray photoelectron spectroscopy (XPS) and so on [18]. From theirreports, SO2 adsorbs molecularly, possibly as a sulte, below approximately 200 C; but at higher temperatures, this surface species is oxidized to sulfates by reduction of CeO2. Under inert conditions in ourstudy, the adsorbed sulfur species is stable to approximately 900 C, ahigher temperature than that at which the sulte decomposes to SO2and O2, indicating that the adsorbed sulfur species should be thesulfates according to the reports above [18].We also investigated the decomposition of the adsorbed SO2under reducing conditions [19,20]. The TPD curves in Fig. 4 wereobtained on the CZ, Ni/CZ, Pt/CZ and Pt/Ni/CZ samples using a N2carrier with 10% H2 after pulsing SO2 (0.50 mL of 0.2% SO2 in N2) at300 C for 5 times. For CZ sample, we observe the formation of H2S(m/e = 34) between 550 and 600 C, which is similar to what Luo[19] had reported. This indicates that the adsorbed SO2 on the CZwould be easily decomposed under reducing atmosphere. For Ni/CZsample, the formation of H2S starts at a lower temperature with amajor peak at ca. 490 C and a small shoulder peak at around 520

and 600 C, which may be associated with the reduction of the

adsorbed SO2 on the CZ and NiO, respectively. Furthermore, fromthe integral of the peak area, the amount of H2S that formed onNi/CZ sample is about 2.2 times than that of CZ, suggesting the releaseof H2S is favored by the presence of Ni under reducing conditions. Infact, we are uncertain whether or not all of the adsorbed sulfurspecies would be decomposed completely from the Ni/CZ sample,but the remaining sulfur species in the CZ sample is certain to bemuch more than that of Ni/CZ. The result for Pt/CZ shows the formation of H2S (m/e = 34) between temperature ranges from 400 to500 C, a much lower temperature than that of without noble metal.It is reported that the formation of H2S is kinetically favored in thepresence of noble metals, which enhances the SO2 reduction rate[20]. So, we can conclude that the lower temperature of thedecomposed of SO2 benets from the presence of noble metal. ForPt/Ni/CZ sample, the formation of H2S starts at a further lower temperature with a major peak at ca. 420 C and a small shoulder peakat around 450 and 550 C, which may be also associated with thereduction of the adsorbed SO2 on the CZ and NiO, respectively. Thefurther lower temperature of the decomposed of SO2 may be benetfrom the strong interaction between Ni and Pt noble metals [16].The amount of H2S that formed on Pt/Ni/CZ sample is about 1.8times than that of Pt/CZ sample, indicating that the release of H2S isfavored by the presence of Ni. Thus it can be concluded that there isstill a small amount of sulfur species remained in the Pt/CZ sample.Obviously, for a long-term accumulation, the remained sulfur specieswill inevitably poison the catalyst. In other words, the sulfur tolerantperformance of the Pt/Ni/CZ catalyst is better than the Pt/CZ catalyst,suggesting that adding Ni to the CZ should reduce the deposition ofsulfur species on the catalyst, and improve the sulfur resistance ofthe catalyst.3.4. Catalytic activity of the catalystsThe C3H8 oxidation conversion as a function of temperature overPt/CZ, Pt/Ni/CZ, SPt/CZ and SPt/Ni/CZ catalysts is shown in Fig. 5.Obviously, the Pt/Ni/CZ sample is more active than Pt/CZ, indicatingthat the introduction of nickel enhances the activity of Pt/CZ catalyst.It is worthwhile to note that it has a strong interaction between Niand Pt noble metals, promoting the catalytic activity of the catalyst.With sulfur poisoned, both of SPt/CZ and SPt/Ni/CZ catalystsshow the decline of the catalytic activity for the C3H8 oxidation. In atypical measurement, the C3H8 oxidation conversion has reachedabove 80% at 400 C over the unpoisoned catalysts. After exposingto 0.2% SO2 in N2 at 300 C for 80 h, the C3H8 oxidation conversionof SPt/Ni/CZ catalyst has slightly reduced to 75%; however, the conversion has dramatically reduced to 40% for the SPt/CZ catalyst.

Fig. 5. The C3H8 oxidation conversion as a function of temperature over Pt/CZ, Pt/Ni/CZ,SPt/CZ and SPt/Ni/CZ catalysts.

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These results have proved the speculation above that the remainedsulfur in the catalysts leads to the sulfur poisoning and the deactivation of the catalysts. Additionally, the amount of adsorbed sulfurspecies on the catalysts after their exposure to the SO2 at 300 C for80 h was estimated by EDS. The results show that the amount of S(wt.%) on the S-CZ, S-Ni/CZ, S-Pt/CZ and S-Pt/Ni/CZ are 0.35%, 0.29%,0.33% and 0.30%, respectively, which indicates that the amount ofadsorbed sulfur species on the catalysts is almost the same. But theamount of S (wt.%) on the regenerated S-CZ, S-Ni/CZ, S-Pt/CZ andS-Pt/Ni/CZ are 0.18%, 0.12%, 0.09% and 0.03%, respectively, which indicates that adding Ni to the CZ should reduce the deposition of sulfurspecies on the catalyst. Obviously, the incorporation of nickel to theCZ has improved sulfur resistance of the catalyst.4. ConclusionsIn summary, the introduction of nickel to the CZ not only enhancesthe surface area of the catalyst, but also decreases its reductiontemperature. The nickel promoted catalyst has a strong interactionbetween Ni and Pt noble metals, promoting the catalytic activity ofthe catalysts. The studies of SO2 absorption and desorption performance of samples indicate that the addition of nickel to the CZ isable to improve the desorption amount of sulfur species under reducing atmosphere. Finally, the sulfur resistance of Pt/Ce0.4Zr0.6O2 catalyst has been improved due to the lower accumulation of sulfurspecies in the nickel promoted catalyst.AcknowledgmentsProject supported by the National Science Foundation for FosteringTalents in Basic Research of the National Natural Science Foundation ofChina (No. J1103303).